Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the blood oxygen saturation, serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heartbeat.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmissivity and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined.
A typical pulse oximeter for measuring the heart rate and the (arterial) blood oxygen saturation of a subject comprises a red LED and an infrared LED as light sources and one photodiode for detecting light that has been transmitted through a body pare of a patient (i.e. through tissue comprised in a body part). Commercially available pulse oximeters quickly switch between measurements at red and an infrared wavelength and thereby measure the transmissivity of the same area or volume of tissue at two different wavelengths. This is referred to as time-division-multiplexing. The transmissivity over time at each wavelength gives the PPG waveforms for red and infrared wavelengths.
One problem with PPG is that ambient light can leak into the sensor and disturb the measurement. This ambient light may originate from the sun or from electrical lamps (which may emit light at the net frequency). Another problem is that noise, especially low frequent noise (e.g. 1/f noise), but also high frequent noise, disturbs the measurement as well. Furthermore, electromagnetic interference may disturb the measurement as well.
In WO 2013/0190423 A1, a photoplethysmographic device and method is disclosed. The disclosed device comprises a light source for emitting light pulses into tissue of a living being, a light sensor for receiving light from said tissue and generating a sensor signal, a filter unit for filtering said sensor signal, said filter unit comprising a switched in-phase low-pass filter for generating an in-phase filter signal and a switched out-of-phase low-pass filter for generating an out-of-phase filter signal, a control unit for controlling said light source and said filter unit such that the in-phase filter is only switched on during a second time period while the light source is switched on and that the out-of-phase filter is switched on during a first and third time period while the light source is switched off, a subtraction unit for subtracting the out-of-phase filter signal from the in-phase filter signal.
However, there is still room for improvements in the area of PPG vital sign detection and vital sign monitoring, in particular with respect to accuracy and reliability, especially if ambient light levels become higher (e.g. due to narrower wrist straps in case of wristwatch device) and/or if LED levels become lower in order to save power.